Field of the Invention
This invention relates to power generation systems, and more particularly to a thermal power generation system using a combustor and a gas turbine.
Description of Related Art
Technological development of thermal power generation systems equipped with a gas turbine which uses fossil fuel, such as natural gas, as a fuel has been encouraged in order to meet the demands to increase system efficiency and to cut NOx and CO2 emissions from the viewpoints of effective use of fossil fuels and the reduction of environmental loads. High system efficiency is achieved by increasing the pressure ratio and combustion temperature. However, increasing the combustion temperature to a certain level or higher causes a reaction between nitrogen and oxygen in the air and thus produces nitride oxides (NOx), which requires various measures to be taken. To cut carbon dioxide (CO2) emission, a separation and capture technology including a chemical adsorption method or the like has gained attention.
One of the techniques of achieving both high efficiency and NOx/CO2 emission cuts is an oxy-fuel combustion technique in which oxygen and fuel undergo a reaction without using the air as a working fluid. In the case where water vapor and CO2 are used as a working fluid, even if the combustion temperature is increased so as to achieve high efficiency, NOx is not emitted and CO2 can be readily captured by condensing the water vapor because the exhaust gas contains only water vapor and CO2. However, the use of water vapor as a main working fluid requires a condenser equivalent in size to a steam turbine used in general coal fired power plants and gas turbine combined cycle power plants, and therefore proposed systems mainly use CO2 to aim at compactness and high-efficiency. Such systems are disclosed in Japanese Unexamined Patent Application Publication Nos. 2001-12213 (Patent Document 1) and 2001-159318 (Patent Document 2), for example.
On the other hand, power generation technologies not using fossil fuel at all such as photovoltaic power generation and wind power generation technologies have been developed and electric power derived from those technologies is recently forming an increasing proportion of electric power composition. However, since the sunlight and wind power are unstable power sources affected by the weather and other factors, power systems using those unstable power sources need to stabilize their electric power supply by flexibility resources such as pumped hydropower generation or thermal power generation. When the stabilization of the electric power supply is made by thermal power generation, the thermal power generation system has to start operation, stop operation, or operate partially in accordance with the increase and decrease in the electric power supply from the photovoltaic or wind power generation system. For excessive electric power supply, techniques to activate only a compressor to store compressed air have been developed, one of which is disclosed in U.S. Pat. No. 4,147,204 (Patent Document 3). In addition, as a technique to store energy more densely and efficiently, methods for generating high pressure carbon dioxide (triple-point CO2) have been proposed instead of compressed air. For example, the methods are disclosed in Japanese Unexamined Patent Application Publication Nos. S63 (1988)-239302 (Patent Document 4) and H03 (1991)-215139 (Patent Document 5).